Support simple A-Z search in Taskbar All Apps.

Supporting the Launcher3 search box is low hanging fruit, so we might as
well do it.

Also renamed the feature flag to be for search in general.

Test: Launching, split screen, and popups work in search.
Bug: 216683257
Flag: ENABLE_ALL_APPS_SEARCH_IN_TASKBAR=false
Change-Id: I056b89be6f458d1d90100e34551baa34037574d1